VoltDB, like all software, uses system resources to perform its tasks. First and foremost, as an in-memory database, VoltDB relies on having sufficient memory available for storing the data and processing queries. However, it also makes use of disk resources for snapshots and caching data for other features, such as export and database replication.

If system resources run low, one or more nodes may fail impacting availability, or worse, causing a service interruption. The best solution for this situation is to plan ahead and provision sufficient resources for your needs. The goal of the VoltDB Planning Guide is to help you do this.

However, even with the best planning, unexpected conditions can result in resource shortages or overuse. In these situations, you want the database to protect itself against all-out failure.

You can do this by setting resource limits in the VoltDB deployment file. System resource limits are set within the <systemsettings> and <resourcemonitor> elements. For example:

<systemsettings>
  <resourcemonitor frequency="30">
     <memorylimit size="70%" alert="60%"/>
     <disklimit>
        <feature name="snapshots" size="75%" alert="60%"/>
        <feature name="droverflow" size="60%"/>
     </disklimit>
  </resourcemonitor>
</systemsettings>

The deployment file lets you set limits on two types of system resources:

For each resource type you can set the maximum size and, optionally, the level at which an alert is sent if SNMP is enabled. In all cases, the allowable amount of the resource to be used can be specified as either a value representing a number of gigabytes or a percentage of the total available. If the limit set by the alert attribute is exceeded and SNMP is enabled, an SNMP alert is sent. If the limit set by the size attribute is exceeded, the database will be "paused", putting it into read-only mode to avoid using any further resources or possibly failing when the resource becomes exhausted. When the database pauses, an error message is written to the log file (and the console) reporting the event. This allows you as the system administrator to correct the situation by reducing memory usage or deleting unnecessary files. Once sufficient resources are freed up, you can return the database to normal operation using the voltadmin resume command.

The resource limits are checked every 60 seconds by default. However, you can adjust how frequently they are checked — to accommodate the relative stability or volatility of your resource usage — using the frequency attribute of the <resourcemonitor> tag. In the preceding example, the frequency has been reduced to 30 seconds.

Of course, the ideal is to catch excessive resource use before the database is forced into read-only mode. Use of SNMP and system monitors such as Nagios and New Relic to generate alerts at limits lower than the VoltDB resource monitor are strongly recommended. And you can integrate other VoltDB monitoring with these monitoring utilities as described in Section 6.3, “Integrating VoltDB with Other Monitoring Systems”. But the resource monitor size limit is provided as a last resort to ensure the database does not completely exhaust resources and crash before the issue can be addressed.

The following sections describe how to set limits for the individual resource types.

<systemsettings>
  <resourcemonitor>
     <memorylimit size="10"/>
  </resourcemonitor>
</systemsettings>

<systemsettings>
  <resourcemonitor>
     <memorylimit size="70%"/>
  </resourcemonitor>
</systemsettings>

<systemsettings>
  <resourcemonitor>
     <memorylimit size="70%" alert="60%"/>
  </resourcemonitor>
</systemsettings>

<systemsettings>
  <resourcemonitor>
     <disklimit>
        <feature name="snapshots" size="200"/>
        <feature name="commandlog" size="70%"/>
     </disklimit>
  </resourcemonitor>
</systemsettings>

<systemsettings>
  <resourcemonitor>
     <disklimit>
        <feature name="snapshots" size="200" alert="150"/>
        <feature name="commandlog" size="70%" alert="60%"/>
     </disklimit>
  </resourcemonitor>
</systemsettings>